In the manufacture of a semiconductor device, such as an IC, LSI, or the like, a plurality of circuit patterns are overlaid on a wafer. Each circuit pattern is drawn on a reticle and is transferred onto a wafer by exposure. In an exposure apparatus which performs exposure, a reticle and a wafer are placed on a reticle stage and a wafer stage, respectively. Alignment is performed such that a circuit pattern on the reticle is transferred to a circuit formation position of the wafer. With this exposure, a new circuit pattern is overlaid on a transferred circuit pattern on the wafer. By repeating this, a circuit having a plurality of overlapping patterns can be formed on the wafer.
In an exposure apparatus, a wafer and a reticle are aligned with each other for each of a plurality of circuit formation positions on the wafer by moving the wafer, i.e., driving a wafer stage in order to transfer a circuit pattern of the reticle to each circuit formation position. To implement this alignment of the wafer and reticle, a reticle stage and the wafer stage are aligned with each other, then a reticle and the reticle stage are aligned with each other, thereby defining a relative positional relationship between the reticle and the wafer stage.
The center position of each circuit pattern drawn on the reticle shifts from the center position of the reticle itself. For this reason, to align the reticle stage and reticle with each other, a reticle alignment mark, which has been drawn on the reticle together with the circuit pattern, is detected, and the reticle is fixed on the reticle stage such that the reticle alignment mark overlaps with a reticle reference mark arranged on the reticle stage.
The reticle is arranged on a reticle chucking pad on the reticle stage and is chucked by the chucking pad, thereby fixing it on the reticle stage. To align the reticle alignment mark and reticle reference mark with each other, there are available the following two arrangements.
(1) A reticle stage, having a mechanism which enables a reticle chucking pad to move on the reticle stage, is used. The reticle chucking pad on the reticle stage is moved while chucking the reticle.
(2) A reticle stage having a fixed reticle chucking pad is used. A reticle is moved by another unit before chucking the reticle with the reticle chucking pad, and a relative position between the reticle and the reticle stage is changed, thereby aligning the reticle. After completion of the alignment, the reticle chucking pad chucks the reticle to fix it on the reticle stage.
In a scanning exposure apparatus, a reticle stage exposes a wafer while scanning it. Providing on the reticle stage a mechanism (the arrangement (1)) which allows the chucking pad to move increases the weight of the reticle stage and the complexity of the apparatus, and, thus, is hard to adopt. For this reason, a scanning exposure apparatus generally adopts an alignment method which uses the arrangement (2).
In, e.g., a reticle 100, as shown in FIG. 5, reticle alignment marks 101, as well as a circuit pattern, are drawn on a reticle pattern surface 104. The reticle 100 is placed such that the position of each of the reticle alignment marks 101 coincides with that of each of reticle reference marks 111, as shown in FIG. 6, and is fixed on a reticle stage 110 by chucking pads 112. FIGS. 5 and 6 show cases wherein a center position 102 of the reticle pattern surface 104 of the reticle 100 coincides with a center position 103 of the reticle itself.
However, if the circuit pattern drawn on the reticle largely shifts from a position where the circuit pattern is to be drawn, the position of the reticle alignment mark also shifts by the shift of the pattern. For this reason, if the reticle is positioned on the reticle stage such that the position of the reticle alignment mark coincides with that of the reticle reference mark, each chucking pad 112 may partially project from the reticle and may be unable to properly chuck or fix the reticle.
For example, if the position where the reticle pattern surface 104 is formed shifts from an intended position, as shown in FIG. 7, a shift occurs between the center 102 of the reticle pattern surface and the center 103 of the reticle 100 itself. This shift directly results in a shift of the reticle alignment mark 101. If the reticle is arranged on the reticle stage 110 such that the reticle alignment mark 101 coincides with the reticle reference mark 111, part of the chucking pad 112 may project from the reticle 100, as shown in FIG. 8.
In this case, if the reticle chucking pad 112 is movable on the reticle stage 110, the chucking pad 112 need only be moved. Otherwise, it cannot chuck the reticle. Granted that the chucking pad 112 can chuck the reticle, its chucking force decreases. The reticle may shift or may become detached from the reticle chucking pad during a reticle scan operation. Detachment of the reticle from the reticle stage or the reticle chucking pad's inability to chuck the reticle during a reticle scan operation is a serious accident for an exposure apparatus. Much labor and expense are required to recover from the accident.
In recent years, a demand for an exposure apparatus having a high throughput leads to a demand for a further increase in scan speed. The area of a reticle chucking pad is increased to augment its chucking force, thereby supporting a high-acceleration reticle stage. This increase in area of the reticle chucking pad narrows the tolerance for a shift of a drawn circuit pattern and increases the complexity of reticle manufacture and management.